1. Technical Field
The disclosed embodiments relate generally to projection of polarization-encoded images and, more specifically, to a polarization conversion system and method for transmitting polarization-encoded imagery to a projection screen.
2. Background
FIG. 1 is a schematic diagram illustrating an exemplary polarization-preserving display system 100. The display system 100 includes a projection screen 102 and polarization filtering eyewear 104. Stereoscopic three-dimensional (3D) imagery is observed using a single polarization-preserving screen 102 sequentially displaying left and right perspective imagery, with polarization filtering eyewear 104. The polarization filtering eyewear 104 contains two lenses 106 and 108 of alternately orthogonal polarization.
3D imagery can be synthesized using polarization control at the projector to encode, and polarization filtering eyewear to decode the left and right perspective imagery (See, e.g., commonly-owned U.S. Pat. No. 4,792,850, entitled “Method and system employing a push-pull liquid crystal modulator,” to Lenny Lipton et al. and U.S. patent application Ser. No. 11/424,087 entitled “Achromatic Polarization Switches,” filed Jun. 14, 2006, both of which are herein incorporated by reference in their entirety for all purposes).
A conventional implementation of polarization control after the projection lens is shown in FIG. 2. Nearly-parallel rays emerge from the output of the lens, appearing to originate from a pupil inside of the lens, and converge to form spots on a distant screen. Ray bundles A, B, and C in FIG. 2 are bundles forming spots at the bottom, center, and top of a projection screen. The light emerging from the projection lens is randomly polarized, depicted in FIG. 2 as both S- and P-polarized light. The light passes through a linear polarizer, resulting in a single polarization state after the polarizer. The orthogonal polarization state is absorbed (or reflected), and the light flux after the polarizer is less than 50% of the original flux (resulting in a dimmer final image). The polarization switch is synchronized with the image frame, and the polarization state emerging from the polarization switch is alternated, producing images of alternately orthogonal polarization at the screen. Polarization selective eyewear 104 allows images of one polarization to pass to the left eye, and images of the orthogonal polarization to pass to the right eye. By presenting different images to each eye, 3D imagery can be synthesized.
This system is currently in use in movie theatres. However, typically, this system design suffers from having more than 50% of the light absorbed by the polarizer, and thus the resulting image is typically more than 50% dimmer than that of a typical 2D theatre. Moreover, time-sequential stereoscopic 3D further reduces the brightness by more than 50%. The dimmer image can therefore limit the size of the theatre used for 3D applications and/or provides a less desirable viewing experience for the audience.